Wave signaling system



April 28, 1936. H A WHEELER 2,039,136

` WAVE SIGNALING SYSTEM Filed May 3l, 1934 INVENTOR HAROLD A WHEELER ATTORNEYS Patented Apr. 28, 1936l "UNITED STATES PATENT orifice Haleltine Corporation poration of Delaware um! (my. N. l acor- Appueenen Mey s1, 1934, serial No. 728,441: e

s claim.. (01.2'50-20) i.

This invention relates primarily to improvements in tuned radio broadcast receivers adapted to reproduce speech or music.

An` object of the invention is to provide an economical but effective means of securing a high degree of fidelity in signal reproduction free of, interference from stations operating on carrier channels adjacent that of the station being received.v Y i A factor which militates against perfect reproduction of speech or music in radio broadcast reception resides in the allocation of broadcasting channels at frequency intervals of only kilocycles. whereas the important frequencies present in speech and music cover a range on the order of this figure. Receiving a band sumciently wide to include sidebands corresponding to the entire range of acoustical frequencies would, therefore, result in interference from the carriers of the adjacent broadcasting channels.

A high degree offidelity in signal reproduction is obtainable by receiving audio frequencies with-V in an 'upper limit of less than 10 kilocycles.l Therefore, to achieve the optimum in ildelityconsistent with avoidance of interference," the selectivity characteristic of a receiver should be of rectangular shapeand such as to provide uniform response throughout a `frequency band of slightly less than 10 kilocycles for single side band and slightly less than kilocycles for double side band reception, with complete suppression or infinite attenuation of frequencies outside this band. f

It is a' relatively difficult and expensive matter to approximate the ideal ,rectangular 'selectivity characteristic in tuned radio or intermediate frequency coupling systems such as are employed in commercial radio broadcast receivers. In general the cutoff provided'by such coupling systems 'is suiilcientlyv gradual that substantial elimination of interferencefrom broadcasting channels adjacent that being received, is secured only at an appreciable sacrice in 'delity of signal repro-s duction due to non-uniform-reception of the carriersidebands. In accordance with the present invention, iidelity in signal rep ction free'from interference is achieved by utilization in the'radio and intermediate frequency portions of a receiver, of tuned -couplings havingsumciently broad band responsetoreceive nearly uniformly carrier sidebands sufficiently wide/to include the important range of frequencies occurring in speech and music. Interference from adjacent broadcasting. 55 channels is then eliminated by interpositionin -The resulting overalls'electivity is, however, in-

the audio frequency circuits of the receiver of one or more lwave traps tuned to the frequency difference between adjacent carrier channels.

In the drawing:.

Fig. 1 is a circuit diagram of a superheterodyne 5 receiver containing the invention.

Fig, 2 illustrates graphically the relative overall response from the antenna circuit to thek loud speaker output of a receiver, such as that of Fig. 1.

Figs. 3 to 5 inclusive, showfmodied forms of 10 wave traps applicable to the loud speaker circuit of the Fig. 1 system.

In the system of Fig. l1, an antenna circuit A is coupled through a tunable selector T1, comprising a transformer having a condenser tuned secondary, to .the input of a stage of radio-frequency amplification, tube V1. The output of tube V1 is in turn coupled through a tunable -selector T2, similar to T1, to the input of a modulator tube V2.

Heterodyne oscillations are applied to the mod- 20 ulator input by means of a resistance R2 through which the cathodes of the modulator tube Va and an oscillation generator, tube Va, are grounded in common. Tube V3 is adapted to generate oscillations of heterodyne frequency by reason of a g5 tuned impedance Tb, grounded at an intermediate point Ge, and connected from the grid of tube Vs through a blocking condenser b to its anode.

The medunted output of tube' ve is selectively applied to a stage of intermediate frequency am- 30 plification, tube vnthrough a selector T4, consisting of a transformer, the primary and Secndary coils 'of which are permanently tuned by the shunt condensers to the modulated or intermediate frequency. A rectifier.' tube Vs, receives g5 the o'utput of 'tube V4 through a permanently tuned selector Ts, similar to T4. The rectified output is applied through a variable tap potentiometer R5, or volume conti- 91, to a stage of audio-frequency amplification, tube 40 Ve, and thence through a stage o f push-pull ampliflcation, comprising tubes Vv and Vs interposed between audio-transformers Tv and Ta, to fan electro-acoustical -device-or loud speaker S.- actuated by a voice coil Le. i, 45

The tunable vselectors T1 and T: and -thepermanently tuned selectors T4 and Te, arebr'oadly -tuned to respond ,nearly uniformly over a frequency band approximately 14 kilocycles wide.

sumcient against interference from .the broad- ,practice at 10 kilocycle intervals from the station selected. This condition is corrected, in accordance with the present invention, by one or 55 casting channels spaced, according to existing more "wave traps W, W1, W2, interposed in audio-frequency circuits of the system. Each of these traps is tuned to 10 kilocycles for suppressing the audible carrier beat notes due to adjacent broadcasting channels.

A'I'he audio-frequency amplifier tube Ve is a pentode. This type of tube is particularly well adapted to the use of traps, such as W1- and W1 in its cathode and anode circuits respectively, because with a pentode or screen grid type of tube, the cathode and anode circuits are nearly independent of each other, and the trap action Y is therefore simple.

In the anti-resonant 10 kilocycle trap W1, interp'osed in the common input and output path electrically linking the cathode, control grid, and anode of the tube Vs, the values of coil L1' and the shunt condenser C1 are such that their reactances at 10 kilocycles are of the same order of magnitude as the mutual resistance (reciprocal of mutual conductance) of tube Vs. The trap action is nearly independent of other properties ofthe tube and associated circuits and functions to suppress a selected high audio-frequency comprised in, or constituting a component of, the

audio-frequency current traversing the common I ter impedance being determined largely by the resistor R1 connected across the secondary Sv and thus effectively shunting the series connected coil Le and condenser C2. This relation, which is made possible by the exceedingly high plate-resistanceof the pentode tube Vs, has the advantage of being independent ,of tube characteristics.

The voice coil trap W, tuned to 10 kilocycles, comprises a coil L, bridged in series with a condenser C across the voice coil La of the speaker S, andv also across. the secondary of the audiotransformer Ts.

L1 denotes secondary leakage inductance lof transformer Ta, which is the apparent secondary inductance measured with the primary short circuited. .The preferred relation between the inductances L1, La and L as measured at a frequency near 10 kilocycles, is:

This gives a cut-off frequency of 7 kilocycles with the trap W tuned, as stated. If L1 and La are unequal, nearly as good results are obtained if:

. LPL,

(2 I L Ll'l-Lz 'I'hat is, L equals the inductance of L1 and La in parallel. None of these relations ls critical except the tuning' of trap W to 10 kilocycles.

Usually only one or two of these three traps W, W1 and W2, shown in Fig. 1, is required. The cathode trap W1 employs reactance values for elements L1' and C1 which are at 10 kilocycles, on the order of 1000 ohms, and are usually most easily and cheaply secured; The voice coil trap W gives the bestjperformance, however, because coil to around 50 percent. ance of the trap accordingly increases with frefrequency range a relatively high efficiency of its cooperation with the inherent inductance,

such as L1 and La, of the circuit.

In Fig. 2, curves A and B show by way of comparison the effect of the voice coil trap W alone, that is, omitting the cathode and plate traps W1 and W1 respectively. These curves represent the overall behavior of the entire receiver, including both the carrier frequency and audiofrequency portions. The dotted curve A, graphically depicting the output with the voice coil trap W omitted, indicates insufficient selectivity against 10 kilocycle beat notes from adjacent carrier channels. Curve B, illustrative of the response with trap W included in the circuit, indicates the greatly improved selectivity against 10 kilocycle beat notes obtained Without impair- 'ing the output over the most useful audio-frequency range of 50 to 7000 cycles; several octaves.

The relationships of Equations (1) and (2) are largely responsible for tne sharp cut-off between 7 and 10 kilocycles represented by curve B.

Fig. 3 shows a modied form of voice coil trap which is preferred when L1 is less than La. The preferred relation of inductances is:

If L1 is greater than L1, the preferred modication is that of Fig. 4 when:

The modification of Fig. 5 permits of the vgreatest flexibility in design, although it is usually unnecessary to refine the circuit to this extent. Assuming the inductances L, L1, and L1 and the resistance Rz of the speaker voice coil to be measured at 7 kilocycles, then the preferred relations are:

None of the above relations is critical, except that L and C must, ask stated, be accurately tuned to 10 kilocycles in all cases.

The voice coll trap W in all of its modifications improves the efficiency of the speaker S with increasing eifectiveness as the cut-off frequency of 7000 cycles is approached.V Below resonance, i. e., below 10 kilocycles, trap W is capacitively reactive and of a magnitude which increases rapidly with approach to resonance. On the other hand the reactance of the voice coil L2 is inductive, and may be such at the higher audio-frequencies around 7000 cycles, as to reduce the power factor of the The capacitive reactquency in such relation to the accompanying variation in inductive reactance of the voice coil as to maintain throughout the active audioof signal energy transfer.

I claim:`

1. 1u a tuned radio broadcast receiving, rectifying and signal reproducing system, means relaying the rectified signals including a vacuum tube having cathode, anode, control grid and screen grid electrodes, a common input and output path electrically linking said cathode and said control grid and anode, and means for suppressing a selected high audible frequency comprised in said path and including a coil and condenser in parallel the component reactances of which are, at said selected frequency, of the same order of magnitude as the mutual resistance of said tube.

2. In a tuned radio broadcast receiving, rectifying and reproducing system, means relaying the rectified signals including a screen grid vacuum tube having an anode and a. cathode, and means for separately selecting and suppressing audible carrier beat notes comprising a resistive impedance effectively shunting a coil and condenser connected in series between said cathode and anode, the component reactances of said coil and condenser being, at the beat note frequency, of the same order of magnitude as said resistive impedance.

3. In a tuned radio broadcast receiving, recti-` fying and reproducing system, an electro-acoustical responsive device having an actuating coil of inductance L2 coupled to said system through a transformer having a secondary leakage inductance L1, a resonant path tuned to a high audible frequency to be suppressed shunting said actuating coil, said path including a condenser in series with a coil of inductance L, the several coils being relatively so proportioned that their inductances eiective near said suppressed frequency satisfy substantially the relation: L1=L2=2L.

4. In a tuned radio broadcast receiving, rectifying and reproducing system, an electro-acoustical responsive device having an actuating coil of inductance La coupled to said system through a transformer having a secondary leakage inductance Li, a resonant path tuned to a high audible frequency to be suppressed shunting said actuating coil, said path including a condenser in series with a coll of inductance L, the several coils being relatively so proportioned that their inductances effective near said suppressed frequency satisfy substantially the relation:

to be suppressed shunting said actuating coil,

said path including a condenser in series with a coil of inductance L, the several coils being relatively so proportioned that their inductances effective near said suppressed frequency satisfy substantially the relation: L1+La=L2=2L.

6. In a tuned radio broadcast receiving, rectifying and reproducing system, an electro-acoustical responsive device having an actuating coil of inductance La connected in series with a coil of inductance L4 to a system through a transformervhaving a secondary leakage inductance L1, a resonant path tuned to a high audible frequency to be suppressed shunting the transformer secondary coil, said path including a condenser in series with a coil of inductance L, the several said coils being relatively so proportioned that their inductances effective near said suppressed frequency satisfy the relation: Li= Lz+L4=2L. l

7. I n a tuned radio broadcast receiving, rectifying and reproducing system, an electro-acoustical responsive device having an actuating coil of inductance La, connected in series with coils of inductances L4 and La respectively to said system through a transformer having a secondary leakage inductance L1, a resonant path tuned to a high audible frequencyto be suppressed, bridged across said actuating coil and the coil of inductance L4, said path including a condenser of capacity C in series with a coil of inductance L, the

several said coils bei'ng relatively so proportioned i that their inductances effective near said suppressed frequency satisfy substantiallythe relation: Li+L2=L3+L4=2L.

8. In a tuned radio broadcast receiving, rectifying and reproducing system, an electro-acoustical responsive device having an actuating coil of inductance L2 and of resistance R2, connected in series with coils of inductances Lr and La respectively, to said system through a transformer having a secondary leakage inductance Li, a resonant path tuned to a high audible frequency to be suppressed, bridged across said actuating coil and the coil of inductance L4, said path including a condenser of capacity C in series with a coil of inductance L, the several said coils and said condenser being relatively so proportioned that said inductances effective near said suppressed frequency satisfy substantially the relations: L1+Ia=Io+L4=2L; and

.RFJEILC HAROLD A. 

